A large diameter optical fiber and a bundle fiber having a plurality of optical fibers bundled together are used as laser light guides for transmitting laser beams having specific wavelength(s), such as those for use in laser scalpels (see Japanese Patent Laid-Open Publication No. 2003-086868), and also used as light guides incorporated in endoscopes. The light guide of the endoscope transmits illumination light to illuminate a body cavity of a patient (see Japanese Patent Laid-Open Publication No. 2000-199864 and U.S. Pat. No. 5,888,194 corresponding to Japanese Patent Laid-Open Publication No. 9-166754). In addition, such optical fibers are used for data signal communications (see U.S. Pat. No. 4,877,300, corresponding to Japanese Patent Laid-Open Publication No. 2-163708).
The light guides described in Japanese Patent Laid-Open Publication No. 2000-199864 and U.S. Pat. No. 5,888,194 are required to illuminate as large an area as possible in the body cavity so as to facilitate finding a lesion. To illuminate a larger area, it is necessary to increase a divergence angle of exit light from the optical fiber.
As described in U.S. Pat. No. 4,877,300 and Japanese Patent Laid-Open Publication No. 2003-086868, tapering a light exit portion of the optical fiber enables to increase the divergence angle of the exit light. However, the use of the tapered optical fiber alone cannot increase the divergence angle enough to facilitate finding a lesion. It should be noted that, in U.S. Pat. No. 4,877,300, the tapered optical fiber is used for improving light coupling efficiency and conversion of a beam diameter. In Japanese Patent Laid-Open publication No. 2003-086868, the tapered optical fiber is used for increasing laser power density.